Lysine and threonine biosynthesis in sorghum seeds: characterisation of aspartate kinase and homoserine dehydrogenase isoenzymes

Aspartate kinase (AK, EC 2.7.2.4) and homoserine dehydrogenase (HSDH, EC 1.1.1.3) have been partially purified and characterised from immature sorghum seeds. Two peaks of AK activity were eluted by anion‐exchange chromatography [diethylaminoethyl (DEAE)‐Sephacel] with 183 and 262 mM KCl, and both activities were inhibited by lysine. Similarly, two peaks of HSDH activity were eluted with 145 and 183 mM KCl; the enzyme activity in the first peak in elution order was shown to be resistant to threonine inhibition, whereas the second was sensitive to threonine inhibition. However, following gel filtration chromatography (Sephacryl S‐200), one peak of AK activity co‐eluted with HSDH and both activities were sensitive to threonine inhibition, suggesting the presence of a bifunctional threonine‐sensitive AK–HSDH isoenzyme with a molecular mass estimated as 167 kDa. The activities of AK and HSDH were studied in the presence of lysine, threonine, methionine, valine, calcium, ethylene glycol bis(2‐aminoethylether)‐N,N,N′N′‐tetraacetic acid, calmodulin, S‐adenosylmethionine (SAM), S‐2‐aminoethyl‐l ‐cysteine (AEC) and increasing concentrations of KCl. AK was shown to be inhibited by threonine and lysine, confirming the existence of two isoenzymes, one sensitive to threonine and the other sensitive to lysine, the latter being predominant in sorghum seeds. Methionine, SAM plus lysine and AEC also inhibited AK activity; however, increasing KCl concentrations and calcium did not produce any significant effect on AK activity, indicating that calcium does not play a role in AK regulation in sorghum seeds. HSDH also exhibited some inhibition by threonine, but the majority of the activity was not inhibited, thus indicating the existence of a threonine‐sensitive isoenzyme and a second predominant threonine‐insensitive isoenzyme. Valine and SAM plus threonine also inhibited HSDH; however, increasing concentrations of KCl and calcium had no inhibitory effect.     

Agrobacterium-mediated transformation of cereals: a promising approach crossing barriers

Cereal crops have been the primary targets for improvement by genetic transformation because of their worldwide importance for human consumption. For a long time, many of these important cereals were difficult to genetically engineer, mainly as a result of their inherent limitations associated with the resistance to Agrobacterium infection and their recalcitrance to in vitro regeneration. The delivery of foreign genes to rice plants via Agrobacterium tumefaciens has now become a routine technique. However, there are still serious handicaps with Agrobacterium -mediated transformation of other major cereals. In this paper, we review the pioneering efforts, existing problems and future prospects of Agrobacterium -mediated genetic transformation of major cereal crops, such as rice, maize, wheat, barley, sorghum and sugarcane.     

Effects of multiple matings on reproductive fitness of male and female Diaeretiella rapae

Mating frequency and the amount of sperm transferred during mating have important consequences on progeny sex ratio and fitness of haplodiploid insects. Production of female offspring may be limited by the availability of sperm for fertilizing eggs. This study examined multiple mating and its effect on fitness of the cabbage aphid parasitoid Diaeretiella rapae McIntosh (Hymenoptera: Aphidiidae). Female D. rapae mated once, whereas males mated with on average more than three females in a single day. The minimum time lag between two consecutive matings by a male was 3 min, and the maximum number of matings a male achieved in a day was eight. Sperm depletion occurred as a consequence of multiple mating in D. rapae. The number of daughters produced by females that mated with multiple‐mated males was negatively correlated with the number of matings achieved by these males. Similarly, the proportion of female progeny decreased in females that mated with males that had already mated three times. Although the proportion of female progeny resulting from multiple mating decreased, the decrease was quicker when the mating occurred on the same day than when the matings occurred once per day over several days. Mating success of males initially increased after the first mating, but then males became ‘exhausted’ in later matings; their mating success decreased with the number of prior matings. The fertility of females was affected by mating with multiple‐mated males. The study suggests that male mating history affects the fitness of male and female D. rapae.     

Co-localisation of host plant resistance QTLs affecting the performance and feeding behaviour of the aphid Myzus persicae in the peach tree

The architecture and action of quantitative trait loci (QTL) contributing to plant resistance mechanisms against aphids, the largest group of phloem-feeding insects, are not well understood. Comparative mapping of several components of resistance to the green peach aphid (Myzus persicae) was undertaken in Prunus davidiana, a wild species related to peach. An interspecific F(1) population of Prunus persica var. Summergrand × P. davidiana clone P1908 was scored for resistance (aphid colony development and foliar damage) and 17 aphid feeding behaviour traits monitored by means of the electrical penetration graph technique. Seven resistance QTLs were detected, individually explaining 6.1-43.1% of the phenotypic variation. Consistency was shown over several trials. Nine QTLs affecting aphid feeding behaviour were identified. All resistance QTLs except one co-located with QTLs underlying aphid feeding behaviour. A P. davidiana resistance allele at the major QTL was associated with drastic reductions in phloem sap ingestion by aphids, suggesting a phloem-based resistance mechanism. Resistance was also positively correlated with aphid salivation into sieve elements, suggesting an insect response to restore the appropriate conditions for ingestion after phloem occlusion. No significant QTL was found for traits characterising aphid mouthpart activity in plant tissues other than phloem vessels. Two QTLs with effects on aphid feeding behaviour but without effect on resistance were identified. SSR markers linked to the main QTLs involved in resistance are of potential use in marker-assisted selection for aphid resistance. Linking our results with the recent sequencing of the peach genome may help clarify the physiological resistance mechanisms.     

Possible Host Adaptation as an Evolution Factor of Cowpea aphid‐borne mosaic virus Deduced by Coat Protein Gene Analysis

Cowpea aphid‐borne mosaic virus (CABMV) causes major diseases in cowpea and passion flower plants in Brazil and also in other countries. CABMV has also been isolated from leguminous species including, Cassia hoffmannseggii, Canavalia rosea, Crotalaria juncea and Arachis hypogaea in Brazil. The virus seems to be adapted to two distinct families, the Passifloraceae and Fabaceae. Aiming to identify CABMV and elucidate a possible host adaptation of this virus species, isolates from cowpea, passion flower and C. hoffmannseggii collected in the states of Pernambuco and Rio Grande do Norte were analysed by sequencing the complete coat protein genes. A phylogenetic tree was constructed based on the obtained sequences and those available in public databases. Major Brazilian isolates from passion flower, independently of the geographical distances among them, were grouped in three different clusters. The possible host adaptation was also observed in fabaceous‐infecting CABMV Brazilian isolates. These host adaptations possibly occurred independently within Brazil, so all these clusters belong to a bigger Brazilian cluster. Nevertheless, African passion flower or cowpea‐infecting isolates formed totally different clusters. These results showed that host adaptation could be one factor for CABMV evolution, although geographical isolation is a stronger factor.     

The relationship between conidial dose,moulting and insect developmental stage on the susceptibility of cotton aphid,Aphis gossypii,to conidia of Lecanicillium attenuatum,an entomopathogenic fungus

The cotton aphid is one of the most serious pests of greenhouse vegetable crops worldwide. It is difficult to control because field populations usually include simultaneously several insect developmental stages. The current research evaluated an isolate (CS625) of Lecanicillium attenuatum, a fungal pathogen of aphids, as to its virulence against different developmental stages of cotton aphid, Aphis gossypii. The influence on mortality of several other factors also was examined: (a) insect moulting, (b) the number of conidia attached to insect cuticles and (c) germination rates of conidia on cuticles of aphids at various developmental stages. Mortality of cotton aphids treated with L. attenuatum conidia varied according to the developmental stage of the host, i.e. the LT_50s with third-instar nymphs and adults was shorter than with first-instar nymphs. The number of spores attached to the surface of first-instar nymphs was approximately one-half of that on third-instar nymphs and adults. Also, the level of spore germination on the surface of first-instar nymphs was lower than on the surface of other stages of the aphid. After moulting, the numbers of conidia attached to new insect cuticles were less than on exuviae. These results suggest that early nymphal stages of cotton aphids may escape fungal disease due, at least in part, to a combination of three factors: low numbers of conidia attached to their cuticles; low levels of conidial germination and rapid ecdyses, which removed conidia before their germ tubes penetrated the host hemolymph.     

Unexpected problem in aphid DNA barcoding by universal primers

DNA barcode (mitochondrial COI) sequences have allowed for species identification of aphids. In this study, we newly found a DNA barcoding problem in a part of the DNA sequences for Sitobion avenae. Five S. avenae individuals showed differences of, on average, 32.60% in the DNA sequences from other conspecific individuals, and a BLAST search revealed that the five sequences are similar to those of aphid parasitoids such as Aphidius, Ephedrus and Praon spp. (Hymenoptera: Braconidae). Based on these results, we concluded that the universal primers used in aphid DNA barcodes can amplify barcode sequences from parasitoid species within host aphids.     

The aphid-tending ant Lasius fuji exhibits reduced aggression toward aphids marked with ant cuticular hydrocarbons

Some aphid species are attended by ants, which protect aphids against enemies, but ants sometimes prey on the aphids they are attending depending on the resource conditions. A previous study indicated that the ant Lasius niger preys less on the aphid individuals that experienced ant attendance than on those that did not. This observation leads to the hypothesis that ants transfer some substances to the aphids they attend and selectively prey on the aphids without the substances. In this study, we focus on cuticular hydrocarbons (CHCs), which are used by ants as nestmate recognition substances, and test whether ants discriminate the aphids on the basis of CHCs. We confirmed that the ant Lasius fuji preyed less on the aphids that were attended by their nestmates than those that were not attended. Glass dummies treated with CHCs from attended aphids were attacked less by ants than those treated with CHCs from non-attended aphids. The CHC profiles of ant attended aphids resembled those of the ants, suggesting that ants’ CHCs are transferred to the aphids’ body surface through ant attendance. These results support the hypothesis that ants “mark” their attended aphids with their CHCs and the CHCs reduce ant predation intensity.